nbd_set_request_block_size - control whether NBD_OPT_GO requests block size


 #include <libnbd.h>

 int nbd_set_request_block_size (
       struct nbd_handle *h, bool request


By default, when connecting to an export, libnbd requests that the server report any block size restrictions. The NBD protocol states that a server may supply block sizes regardless of whether the client requests them, and libnbd will report those block sizes (see nbd_get_block_size(3)); conversely, if a client does not request block sizes, the server may reject the connection instead of dealing with a client sending unaligned requests. This function makes it possible to test server behavior by emulating older clients.

Note that even when block size is requested, the server is not obligated to provide any. Furthermore, if block sizes are provided (whether or not the client requested them), libnbd enforces alignment to those sizes unless nbd_set_strict_mode(3) is used to bypass client-side safety checks.


If the call is successful the function returns 0.


On error -1 is returned.

Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.

The following parameters must not be NULL: h. For more information see "Non-NULL parameters" in libnbd(3).


nbd_set_request_block_size can be called when the handle is in the following states:

 │ Handle created, before connecting   │ ✅ allowed              │
 │ Connecting                          │ ❌ error                │
 │ Connecting & handshaking (opt_mode) │ ✅ allowed              │
 │ Connected to the server             │ ❌ error                │
 │ Connection shut down                │ ❌ error                │
 │ Handle dead                         │ ❌ error                │


This function first appeared in libnbd 1.12.

If you need to test if this function is available at compile time check if the following macro is defined:



nbd_create(3), nbd_get_block_size(3), nbd_get_request_block_size(3), nbd_set_full_info(3), nbd_set_strict_mode(3), libnbd(3).


Eric Blake

Richard W.M. Jones


Copyright Red Hat


This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA